Field
Embodiments of the present invention generally relate to apparatuses for supporting an exchangeable object, and more particularly to apparatuses and methods for supporting and exchanging an object, for example, a patterning device or substrate, for use in a lithographic apparatus.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, for example, a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (for example, including part of, one, or several dies) on a substrate (for example, a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Manufacturing ICs sometimes requires imaging different patterns on different layers of ICs using different patterning devices, for example, reticles. Accordingly, patterning devices must be exchanged during the lithographic process. Some lithographic apparatuses include a patterning device handling apparatus that exchanges the patterning devices. To load the patterning device on a patterning device support of the lithographic apparatus, a gripping device of the patterning device handling apparatus couples with the patterning device, for example, by using vacuum suction. Then, the patterning device handling apparatus moves the patterning device (for example, by rotating about a turret) towards the patterning device support to load the patterning device.
Generally, the gripping device lowers the patterning device onto the patterning device support. While the gripping device is still coupled to the patterning device, the patterning device support clamps, for example, by creating a vacuum, the patterning device to a surface. The gripping device then releases the patterning device, leaving the patterning device loaded on the patterning device support.
During the loading sequence, the patterning device may accumulate internal mechanical stresses. For example, geometrical non-conformity between the vacuum pads of the gripping device and the surface of the patterning device can deform the patterning device to create stresses. The weight of the gripper can also deform the patterning device when placed on the patterning device support. Internal stresses can also be created by manufacturing tolerances, damage, or wear to the clamping interfaces of the patterning device support. Contamination can also create internal stresses in the patterning device. Contamination particles between the clamping interfaces and the patterning device can deform the patterning device. Because the patterning device support couples with the patterning device while the gripper is also coupled to the patterning device, these internal stresses in the patterning device cannot be partially or fully relieved when transferred to the patterning device support. Stresses or deformations in the patterning device can cause non-deterministic distortions that can lead to overlay errors. Internal stress in the patterning device can be difficult to reproduce between load-to-load, reticle-to-reticle, gripper-to-gripper, and machine-to-machine. So internal patterning device stress generally hurt reproducibility.